postgresql

History

Tom Lane 7b6610508d Apply a better fix to mdunlinkfork(). Replace the stopgap fix I made in `0e758ae89` with a cleaner one. The real problem with `4ab5dae94` is that it contorted this function's logic substantially, by introducing a third code path that required different behavior in the function's main loop. That seems quite unnecessary on closer inspection: the new IsBinaryUpgrade case can just share the behavior of the other immediate-unlink cases. Hence, revert `4ab5dae94` and most of `0e758ae89` (keeping the latter's save/restore errno fix), and add IsBinaryUpgrade to the set of conditions tested to choose immediate unlink. Also fix some additional places with sloppy handling of errno, to ensure we have an invariant that we always continue processing after any non-ENOENT failure of do_truncate. I doubt that that's fixing any bug of field importance, so I don't feel it necessary to back-patch; but we might as well get it right while we're here. Also improve the comments, which had drifted a bit from what the code actually does, and neglected to mention some important considerations. Back-patch to v15, not because this is fixing any bug but because it doesn't seem like a good idea for v15's mdunlinkfork logic to be significantly different from both v14 and v16. Discussion: https://postgr.es/m/3797575.1667924888@sss.pgh.pa.us		2022-11-09 14:15:38 -05:00
..
Makefile	Split all OBJS style lines in makefiles into one-line-per-entry style.	2019-11-05 14:41:07 -08:00
README	Drop the vestigial "smgr" type.	2019-03-07 15:44:04 +13:00
md.c	Apply a better fix to mdunlinkfork().	2022-11-09 14:15:38 -05:00
smgr.c	Rethink PROCSIGNAL_BARRIER_SMGRRELEASE.	2022-05-07 16:32:10 +12:00

README

src/backend/storage/smgr/README

Storage Managers
================

In the original Berkeley Postgres system, there were several storage managers,
of which only the "magnetic disk" manager remains.  (At Berkeley there were
also managers for the Sony WORM optical disk jukebox and persistent main
memory, but these were never supported in any externally released Postgres,
nor in any version of PostgreSQL.)  The "magnetic disk" manager is itself
seriously misnamed, because actually it supports any kind of device for
which the operating system provides standard filesystem operations; which
these days is pretty much everything of interest.  However, we retain the
notion of a storage manager switch in case anyone ever wants to reintroduce
other kinds of storage managers.  Removing the switch layer would save
nothing noticeable anyway, since storage-access operations are surely far
more expensive than one extra layer of C function calls.

In Berkeley Postgres each relation was tagged with the ID of the storage
manager to use for it.  This is gone.  It would be probably more reasonable
to associate storage managers with tablespaces, should we ever re-introduce
multiple storage managers into the system catalogs.

The files in this directory, and their contents, are

    smgr.c	The storage manager switch dispatch code.  The routines in
		this file call the appropriate storage manager to do storage
		accesses requested by higher-level code.  smgr.c also manages
		the file handle cache (SMgrRelation table).

    md.c	The "magnetic disk" storage manager, which is really just
		an interface to the kernel's filesystem operations.

Note that md.c in turn relies on src/backend/storage/file/fd.c.


Relation Forks
==============

Since 8.4, a single smgr relation can be comprised of multiple physical
files, called relation forks. This allows storing additional metadata like
Free Space information in additional forks, which can be grown and truncated
independently of the main data file, while still treating it all as a single
physical relation in system catalogs.

It is assumed that the main fork, fork number 0 or MAIN_FORKNUM, always
exists. Fork numbers are assigned in src/include/common/relpath.h.
Functions in smgr.c and md.c take an extra fork number argument, in addition
to relfilenode and block number, to identify which relation fork you want to
access. Since most code wants to access the main fork, a shortcut version of
ReadBuffer that accesses MAIN_FORKNUM is provided in the buffer manager for
convenience.